Time stretching of audio tracks can be easily done by either interpolating missing samples (slowing down the track), or by throwing away samples (speeding up the track). A drawback is that this results in a pitch change. In order to overcome these issues, we created a time stretcher that would not alter the pitch when the playback speed changed. In this talk we discuss how we created a fast, high quality time stretcher, which is now an integral part of BpmDj. We explain how a sinusoidal model is extracted from the input track, its envelope modeled and then used to synthesize a new audio track. The synthesis timestretches the envelope of all participating sines, yet retains the original pitch. The resulting time stretcher uses only a frame overlap of 4, which reduces the amount of memory access and computation compared to other techniques.
We assume the listener will have a notion about Fourier analysis. We do however approach the topic equally from an educational as well as from a research perspective.
High resolution slides are available at http://werner.yellowcouch.org/Papers/sha2017/index.html